adpcm: Avoid reading out of bounds in the IMA QT trellis encoder

This was broken in 095be4fb - samples+ch (for the previous
non-planar case) equals &samples_p[ch][0]. The confusion
probably stemmed from the IMA WAV case where it originally
was &samples[avctx->channels + ch], which was correctly
changed into &samples_p[ch][1].

CC: libav-stable@libav.org
Signed-off-by: Martin Storsjö <martin@martin.st>